The present invention relates generally to magnetic resonance (MR) imaging, and more particularly to, a method and apparatus to acquire MR images with improved image signal and contrast using a non-selective and notched RF saturation pulse in MR perfusion imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Myocardial perfusion imaging includes the detection of a contrast agent as it passes through muscle tissue in the heart to non-invasively study blood flow in the microcirculation of the heart. Typically, perfusion imaging consists of using an injected contrast agent (bolus) with rapid imaging during the first pass of the bolus using carefully optimized pulse sequence parameters. Quantification of blood flow from these images is accomplished with a region-of-interest based signal, time-intensity curve analysis. To avoid cardiac motion artifacts, the perfusion images are typically acquired with ECG gating to synchronize the repeated acquisition of images at different spatial locations, each to the same relative point in the cardiac cycle. In the past, the period of image acquisition was typically several minutes long, causing the images to suffer from significant respiratory motion artifacts. Such artifacts would require the manual registration and analysis of the perfusion images—a cumbersome and time-consuming task because the user must carefully arrange each image to compensate for the respiratory motion before proceeding to a region-of-interest, time-intensity analysis. Furthermore, the passage of the contrast agent takes place over a temporal span of several seconds. By averaging over several seconds or minutes, the effectiveness of measuring any change in perfusion is severely compromised.
The goal of myocardial perfusion imaging is to detect and characterize the abnormal distribution of myocardial blood flow. The ability to extract quantitative perfusion indices such as time-to-peak, contrast enhancement ratio, and the slope from the first-pass contrast-enhanced MR images requires a generation of myocardial and blood-pool time-intensity curves for desired regions-of-interest. The computation of these curves is complicated when patients do not suspend respiration adequately, which then results in an image mis-registration over time. Mis-registration artifacts occur frequently due to the fact that the breath-hold duration required to capture first-pass kinetics is typically 20-30 seconds. An accurate spatial alignment of images over a period of time is necessary for creating representative and accurate time-intensity curves for a given region of the myocardium.
Moreover, quantification of the blood flow or perfusion to cardiac tissue is also an important consideration. For such quantification, it is essential to acquire images at a specific slice location in order to measure the signal intensity as a function of passage of the contrast media (i.e., time). This calibration slice allows the translation of signal intensity into a contrast media concentration. The input function or the amount of contrast introduced into the cardiac tissue can be extracted from this measurement. Thus, by measuring both the input function at a specific slice location and also the signal intensity variation in the cardiac (myocardial) tissue, the blood perfusion to specific regions of the heart can be computed.
The imaging of blood perfusion in tissue is closely related to the imaging of blood flow in vascular structures, such as in MR angiography. As with MR angiography, MR perfusion imaging is performed by injecting the bolus of an MR active contrast agent into the patient during an imaging session. These agents can either decrease the T1 of blood to enhance the detected MR signal, or decrease the T2 of blood to attenuate the detected MR signal. As the bolus passes through the body, the enhanced or attenuated signal increases or decreases the signal intensity observed in perfused tissue, but not in non-perfused tissue. The degree of signal change in the observed tissue can be used to determine the degree of tissue perfusion. Since perfusion measurements are based on the strength of the MR signals acquired during the scan, it is important that the MR signal strength be made insensitive to other measured variables. One such variable is the magnitude of the longitudinal magnetization Mz, which is tipped into the transverse plane by the RF excitation pulse in the MR pulse sequence. After each such excitation, the longitudinal magnetization is reduced and then recovers magnitude as a rate determined by the T1 constant of the particular spins being imaged. If another pulse sequence is performed before the longitudinal magnetization has recovered, the magnitude of the acquired MR signal will be less than the signal produced by a pulse sequence which is delayed long enough to allow full recovery of the longitudinal magnetization. It is therefore important in perfusion imaging that the longitudinal magnetization variable be maintained at a constant level throughout the scan. One method to maintain a constant signal intensity level regardless of the preceding time is to use a saturation or 90-degree magnetization preparation that allows the same available longitudinal magnetization for a given post-saturation delay time (TI).
Quantitative analysis of myocardial perfusion requires that adequate spatial coverage be maintained, good signal-to-noise (SNR) ratio be attained so that the myocardial perfusion defects can be qualitatively and quantitatively assessed, and that the measurement of the signal in the blood pool reflects contrast agent concentration. That is, it is desired that the MR signal from the blood pool be linear with the concentration of contrast agent in the blood pool. One particular method of myocardial perfusion data acquisition is characterized by a notched perfusion acquisition. It has been shown that this particular method provides good spatial coverage of the myocardium with high image SNR through longer magnetization recovery time (TI) and the application of a saturation recovery preparation RF pulse. However, in some circumstances, this method has been shown to be inadequate because the notched RF pulse saturates blood outside of the target slice but does not perturb (blood) spins within the target slice. As such, within any one target slice, the blood pool signal in the ventricle or aorta will be a combination of saturated and unsaturated blood. This will hinder an accurate measurement of contrast agent concentration as the signal intensity variation will no longer reflect a linear variation of contrast agent concentration. Notwithstanding the drawbacks of this notched approach, it is preferable over a data acquisition method that does not use notched RF pulses or any slice selective saturation. This is because the signal intensity in the myocardial tissue is unaffected by the inflow effects. Absent notched RF pulses or slice selective saturation, a longer physical TI time must be used thereby increasing combined preparation and readout times. Increasing the preparation and readout times reduces the overall number of slices that may be acquired within a single R—R interval or several R—R intervals thereby negatively affecting patient throughput.
It would therefore be desirable to have a means for acquiring MR perfusion images whereby a notched acquisition may be used for improved image SNR and spatial coverage and yet provide a simultaneous measurement of the blood pool signal which may be analyzed to quantify contrast agent concentration. It would be further desirable to have a pulse sequence that reduces variability in blood pool signal such that a linear measurement of contrast concentration in a blood pool may be obtained.